Accumulation of Nutrients, Heavy Metals, and Persistent Organic Pollutants in Bioretention under Stormwater Long-Term Infiltration and Their Relations with Enzyme Activity

Bioretention facilities can effectively control urban runoff pollution. However, in cases of prolonged concentrated infiltration of runoff, the accumulation of pollutants, especially persistent organic pollutants (POPs) in bioretention facilities and their impact on soil enzyme activity remain poorly understood. This paper reported on a field study that analyzed pollutant concentration in the media at 0–10 cm, 20–30 cm, and 40–50 cm depths and their intrinsic relationship with enzyme activities at four facilities that had been in operation for 7–9 years. The results showed that the media surface generally exhibited higher levels of TOC, TN, ${\mathrm{NH}}_3\text{-}\mathrm{N}$, Cu, Zn, Cd and typical POPs compared to a reference point without concentrated rainfall infiltration, showing a significant enrichment phenomenon. In terms of POPs, the concentrations of ${\sum }_{16}\mathrm{PAHs}$, ${\sum }_{12}\mathrm{PCBs}$, and $\sum 8\mathrm{OCPs}$ in the filter media were 9,270.6–25,424.8, 33.4–75.6, and $62.1–163.0\mathrm{\mu g}/\mathrm{kg}$, respectively. However, only PCB126 concentration was close to the screening value specified by the Ministry of Ecology and Environment of China. Except for TP and ${\mathrm{NO}}_3$-N, pollutant concentrations in the media surface were higher and decreased with increasing depth from the surface. The accumulation of pollutants inhibited enzyme activity. Specifically, the activities of invertase, urease, acid phosphatase, and dehydrogenase in the media surface were $4.2–15.4\mathrm{mg}/\mathrm{g}$, $2.0–4.0\mathrm{mg}/\mathrm{g}$, $2.4–4.0\mathrm{mg}/\mathrm{g}$, and $216.7–297.1\mathrm{\mu g}/\mathrm{g}$, respectively. In contrast, the enzyme activities in the middle and deep layers were only 8.3%–36.9% of those found in the media surface. The enzymes activities were relatively lower in the facilities receiving road flow, and were also 1.5%–44.5% lower compared to the reference sites. Correlation analysis revealed a positive and robust relationship between four enzymes activities and TOC and TN. The activities of invertase and urease were positively correlated with Zn, while the activities of invertase and dehydrogenase exhibited significant negative correlations with PCBs and OCPs ($P<0.01$). Enzyme activity can be used to monitor contamination in bioretention facilities to a certain extent.